Low pressure carbon dioxide removal from the anode exhaust of a fuel cell

ABSTRACT

A fuel cell system for removing CO 2  from anode exhaust gas includes a fuel cell having an anode that outputs anode exhaust including H 2 , CO, CO 2 , and water; a shift reactor that receives a first portion of the anode exhaust and performs a water-gas shift reaction to produce an output stream primarily including H 2  and CO 2 ; an anode gas oxidizer (AGO); and an absorption system including an absorber column that absorbs the CO 2  from the output stream in a solvent and outputs a resultant gas including H 2  and a hydrocarbon that is at least partially recycled to the anode, and a stripper column that regenerates the solvent and outputs a CO 2 -rich stream. The AGO is configured to oxidize at least a portion of the CO 2 -rich stream and an AGO input stream that includes one of a second portion of the anode exhaust or a portion of the output stream.

FIELD

The present disclosure relates to fuel cells. In particular, the presentdisclosure relates to a system and method for removing carbon dioxidefrom the anode exhaust of a fuel cell. The systems and methods describedin the present disclosure may be used with any type of fuel cells, andparticularly with high temperature fuel cells such as molten carbonatefuel cells and solid oxide fuel cells.

BACKGROUND

A fuel cell is a device which uses an electrochemical reaction toconvert chemical energy stored in a fuel such as hydrogen or methaneinto electrical energy. Generally, a fuel cell has an anode, a cathode,and an electrolyte layer that together drive chemical reactions thatproduce electricity. Anode exhaust, which may comprise a mixture ofhydrogen, carbon monoxide, and carbon dioxide, is produced as abyproduct from the anode of the fuel cell. The anode exhaust containsuseful byproduct gases such as hydrogen and carbon monoxide, which canbe exported as syngas for other uses, such as fuel for the fuel cell orfeed for other chemical reactions. However, to prepare the anode exhaustto be suitable for such uses, the carbon dioxide present in the anodeexhaust must be removed.

A need exists for improved technology, including a fuel cell system andoperation method in which anode exhaust is recycled to improve theefficiency of a high temperature fuel cell and to increase the overallfuel utilization.

SUMMARY

In certain embodiments, a fuel cell system for removing carbon dioxidefrom anode exhaust gas comprises a fuel cell (e.g., a molten carbonatefuel cell or a solid oxide fuel cell) having an anode configured tooutput an anode exhaust gas comprising hydrogen, carbon monoxide, carbondioxide, and water. The system further includes a shift reactorconfigured to receive a first portion of the anode exhaust gas and toperform a water-gas shift reaction to produce an output stream primarilycomprising hydrogen and carbon dioxide; an anode gas oxidizer; and anabsorption system configured to receive the output stream from the shiftreactor. The absorption system includes an absorber column configured toabsorb the carbon dioxide from the output stream in a solvent and tooutput a resultant gas comprising mainly hydrogen with CO, CO₂ and somehydrocarbon that is at least partially recycled to the anode; and astripper column configured to regenerate the solvent and to output acarbon dioxide-rich stream to the anode gas oxidizer. The absorptionsystem may be an amine absorption system or a physical solvent absorbersystem. The anode gas oxidizer is configured to receive and oxidize ananode gas oxidizer input stream and at least a portion of the carbondioxide-rich stream. The anode gas oxidizer input stream comprises oneof a second portion of the anode exhaust gas or a portion of the outputstream from the shift reactor.

In one aspect, a portion of the carbon dioxide rich stream that is notfed to the anode gas oxidizer may be captured.

In one aspect, part or all of the hydrogen rich gas from the absorbermay be exported and/or recycled.

In one aspect, the shift reactor may be eliminated and H₂+CO may berecycled or exported instead of low purity H₂.

In one aspect, the anode gas oxidizer may be configured to oxidize thesecond portion of the anode exhaust gas to produce hot oxidant gas to bereceived at a cathode.

In one aspect, the amount of gas sent to the absorption system isdetermined by the amount of gas sent to the anode gas oxidizer, which isneeded to provide the heat required by the overall system.

In one aspect, the solvent may include an amine solution.

In one aspect, the solvent may include mixtures of dimethyl ethers ofpolyethylene glycol. These solvents may operate at higher pressure, butmay require less heat to regenerate.

In one aspect, the system may include a first heat exchanger locatedupstream of the shift reactor, the first heat exchanger configured tocool the first portion of the anode exhaust gas; and a second heatexchanger located downstream of the shift reactor, the second heatexchanger configured to cool the output stream.

In one aspect, the system may include a water recovery system downstreamof the second heat exchanger, the water recovery system configured torecover water from the cooled output stream and to recycle the recoveredwater to the anode;

In one aspect, the anode may be configured to receive a fuel gascomprising the resultant gas from the absorber column, the recoveredwater from the water recovery system, and a hydrocarbon streamcomprising at least one of methane, natural gas, propane or otherhydrocarbon.

In one aspect, the system may include a pressure swing adsorption systemconfigured to receive at least a portion of the resultant gas from theabsorber column and to separate hydrogen from the resultant gas. Thepressure swing adsorption system may be configured to output a firststream comprising the hydrogen and a second stream comprising mainlycarbon dioxide, CO, and unrecovered H₂ with some the hydrocarbon. Thesecond stream may be recycled to the anode or sent to the anode gasoxidizer.

In one aspect, the anode gas oxidizer may be configured to receive apre-heated air stream.

In one aspect, the system may include a cathode configured to output acathode exhaust gas. The cathode exhaust gas may be configured to heatan air stream to produce the pre-heated air stream, and/or to heat thefuel gas upstream of the anode and/or heat the stripper bottom toproduce a lean (low CO₂ containing) solvent.

In certain embodiments, a method of removing carbon dioxide from fuelcell anode exhaust gas includes outputting anode exhaust gas comprisinghydrogen, carbon monoxide, carbon dioxide, and water from the anode;receiving a first portion of the anode exhaust gas in a shift reactor;receiving an anode gas oxidizer input stream in an anode gas oxidizer;performing a water-gas shift reaction in the shift reactor to produce anoutput stream primarily comprising hydrogen and carbon dioxide;receiving the output stream from the shift reactor in an absorptionsystem comprising an absorber column having a solvent therein and astripper column; absorbing the carbon dioxide from the output stream inthe solvent and outputting, from the absorber column, a resultant gascomprising mainly hydrogen with some CO₂, CO and hydrocarbon that is atleast partially recycled to the anode; regenerating the solvent andoutputting, from the stripper column, a carbon dioxide-rich stream; andoxidizing the anode gas oxidizer input stream and at least a portion ofthe carbon dioxide-rich stream to produce an oxidant gas. The anode gasoxidizer input stream comprises one of a second portion of the anodeexhaust gas or a portion of the output stream from the shift reactor.The solvent may include an amine solution or mixtures of dimethyl ethersof polyethylene glycol.

In one aspect, the method may include capturing a portion of the carbondioxide-rich stream that is not fed to the anode gas oxidizer.

In one aspect, the method may include cooling the first portion of theanode exhaust gas prior to entering the shift reactor; and cooling theoutput stream of the shift reactor.

In one aspect, the method may include recovering water from the cooledoutput stream; and recycling the recovered water to the anode.

In one aspect, the method may include receiving a fuel gas at the anodethat comprises the resultant gas from the absorber column, the recoveredwater from the water recovery system, and a hydrocarbon streamcomprising at least one of methane, natural gas, propane or otherhydrocarbon.

In one aspect, the method may include heating the fuel gas, upstream ofthe anode, using cathode exhaust gas.

In one aspect, the method may include separating hydrogen from theresultant gas using a pressure swing adsorption system.

In one aspect, the method may include outputting, from the pressureswing adsorption system, a first stream comprising hydrogen; outputting,from the pressure swing adsorption system, a second stream comprisingcarbon dioxide and the hydrocarbon; and recycling the second stream tothe anode.

In one aspect, the method may include heating an air stream usingcathode exhaust gas to form a pre-heated air stream; and providing thepre-heated air stream to the anode gas oxidizer.

In one aspect, the method may not include a shift reactor and H₂+CO isrecycled or exported instead of low purity H₂.

The foregoing is a summary of the disclosure and thus by necessitycontains simplifications, generalizations, and omissions of detail.Consequently, those skilled in the art will appreciate that the summaryis illustrative only and is not intended to be in any way limiting.Other aspects, features, and advantages of the devices and/or processesdescribed herein will become apparent in the detailed description setforth herein and taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a high temperature fuel cell system having anabsorption system configured to remove and recover carbon dioxide fromanode exhaust gas.

FIG. 2 is the same as FIG. 1, except all the anode gas is sent to theshift/water recovery/blower system and part of the gas from the bloweris sent to the anode gas oxidizer (AGO).

FIG. 3 illustrates the high temperature fuel cell system of FIG. 1further including a pressure swing adsorption (PSA) system configured toseparate and recover a higher purity hydrogen gas from anode exhaustgas.

FIG. 4 illustrates the operation of a molten carbon fuel cell that maybe used in the system of FIG. 1, FIG. 2, or FIG. 3.

FIG. 5 is a table providing examples of compositions, temperatures andpressures of various gas streams in the system of FIG. 2.

DETAILED DESCRIPTION

Before turning to the figures, which illustrate the exemplaryembodiments in detail, it should be understood that the claimed systemsand methods are not limited to the details or methodology set forth inthe description or illustrated in the figures.

FIG. 1 illustrates a high temperature fuel cell system 100 according toone embodiment. The high temperature fuel cell system 100 includes atleast one high temperature fuel cell having an anode 126 and a cathode130. The fuel cell may be a Molten Carbonate Fuel Cell (“MCFC”), a SolidOxide Fuel Cell (“SOFC”) or other type of fuel cell.

A hydrocarbon stream 142, a water stream 104, and an stream 124including hydrogen, carbon dioxide, and a hydrocarbon (e.g., methane),are combined and heated to form a anode input stream 128 provided to aninput of the anode 126. The hydrocarbon stream 142 comprises at leastone of methane, natural gas, propane or other hydrocarbon. In the anodeinput stream 128, water from the water stream 104 is present in the formof steam.

Operation of the high temperature fuel cell produces an anode exhaustgas 132 comprising mostly carbon dioxide (40 to 80 mole % on a drybasis), water, hydrogen, and carbon monoxide. The anode exhaust gas 132may be divided into a first portion 136 that is cooled and undergoes awater-gas shift reaction in a shift reactor 110, and a second portion137 that is fed to an anode gas oxidizer (AGO) 133 either separate fromor together with a CO₂-rich stream 118 from a stripper column 115 in aAGO input stream 138 (discussed in further detail below). In operation,all of the anode exhaust gas 132 may be provided as the first portion136, all of the anode exhaust gas 132 may be provided as the secondportion 137, or part of the anode exhaust gas 132 may be provided as thefirst portion 136 with the remainder of the anode exhaust gas 132provided as the second portion 137. Typically, the split is adjusted tomaintain a predetermined cathode inlet temperature to the fuel cell.

Along with the AGO input stream 138 (i.e., the second portion 137 andthe CO₂-rich stream 118), a pre-heated air stream 135 is also fed to theAGO 133. The AGO 133 oxidizes the AGO input stream 138 to produce hightemperature oxidant gas (AGO exhaust stream 139) suitable for use in thecathode 130. By pre-heating the air stream 134 prior to inputting thepre-heated air stream 135 into the AGO 133, the AGO 133 is capable ofoperating at the desired temperature with a smaller amount of the secondportion 137 of the anode exhaust stream. This allows a greater amount ofH₂ recycle, which increases the system efficiency.

The cathode 130 produces a cathode exhaust gas 140 which is vented as avent stream 145 from the high temperature fuel cell system 100 afterheat is recovered and used to pre-heat the air stream 134, thehydrocarbon stream 142, the water stream 104, and/or the stream 124. Inthis example, the recovered heat is also used to heat the strippercolumn 115.

Non-limiting examples of the composition, temperature and pressure ofthe hydrocarbon stream 142, anode exhaust gas 132, the stream 124, theAGO input stream 138, the anode input stream 128, the pre-heated airstream 135, and the vent stream 145 are provided in FIG. 5.

The anode exhaust gas must be cooled and most of the water condensedprior to being fed to an absorber column 114 for amine absorption. Inparticular, the first portion 136 of the anode exhaust gas 132 iscooled, shifted, and cooled further before being sent to the absorbercolumn 114. Typically, the anode exhaust is cooled to 400° F. at theinlet to the shift unit. To maximize CO conversion, a low temperature isfavored by the equilibrium of the reaction, but the reaction kineticslimit how low the gas can be cooled and still react in a reasonablecatalyst volume. The shift reaction is exothermic and the temperaturewill increase 75° F. to 150° F. in the reactor. For maximum conversion,two stages of shift catalyst are used with cooling in between thestages. In particular, the first portion 136 is cooled and undergoes awater-gas shift reaction in the shift reactor 110, whereby the carbonmonoxide and steam are further reacted according to Equation (4) below,using a catalyst which does not enable reforming to produce carbondioxide and more hydrogen. Increasing the CO₂ helps to increase theremoval or carbon from the recycle and/or export gas. Examples of shiftcatalysts that may be used include, but are not limited to, JohnsonMatthey KATALCO shift catalysts, BASF CO-Shift catalysts, and ClariantShiftMax catalysts.

CO+H₂O↔CO₂+H₂  (4)

The stream exiting the shift reactor 110 is further cooled to condensethe remaining water from the gas, and condensed water is removed fromthe gas using a knock out pot or other water separation device toproduce a stream 111 having a composition primarily of hydrogen andcarbon dioxide and a residual of mostly unconverted carbon monoxide andunreacted methane. A typical gas composition for a molten carbonate fuelcell (MCFC) is shown in FIG. 5. For an SOFC, the CO₂ dry mole percentageis approximately 50% instead of 70% for MCFC. The cooling steps upstreamand downstream of the shift reactor 110 may be performed, for example,via an upstream heat exchanger and a downstream heat exchanger. Some orall of the removed water is recycled as the water stream 104 with excesswater being optionally removed from the system. The stream 111 is inputinto a blower B, which outputs a pressurized absorber feed gas 112having a pressure greater than the pressure of the stream 111. For amineabsorber systems, the blower outlet is typically 3 to 5 psig. Thisprovides enough pressure to send the gas through the absorber and thefuel cell. For a physical solvent absorber system, a higher pressure maybe required, 100-300 psig. The shift reactor 110 is optional, butreduces the amount of carbon recycled to the anode feed and allows aslightly higher amount of anode exhaust to be recycled.

The system of FIG. 2 is similar to the system of FIG. 1, except that inFIG. 2, all of the anode exhaust gas 132 is shifted by the shift reactor110, and the gas output by the blower B is divided into the pressurizedabsorber feed gas 112 and the AGO input stream 138. The system of FIG. 2has the advantage of recovering more water, allows control of the anodepressure using the blower, and reduces the amount of water sent to thecathode, which increases the cell life. The recovery of water has theside benefit of making the system independent of water during normaloperation.

The pressurized absorber feed gas 112 is fed to the absorber column 114,which is configured to remove a bulk amount of carbon dioxide containedin the pressurized absorber feed gas 112. In one example, a solvent ofthe absorber column 114 absorbs the carbon dioxide in the pressurizedabsorber feed gas 112 at relatively low pressure (e.g., approximately 17to 20 psia). The resultant gas is output from the absorber column 114 asthe stream 124 including hydrogen, carbon dioxide, and methane.Recycling the anode exhaust (after carbon dioxide removal by theabsorber column 114) to the fuel cell via the stream 124 reduces theamount of fuel needed, thereby increasing the efficiency of thehigh-temperature fuel cell system 100.

The solvent having carbon dioxide absorbed therein is output as a liquideffluent stream 117 to a heat exchanger, heated, and supplied to thestripper column 115. The pressure of the stripper column 115 is lowerthan the pressure of the absorber column 114 such that the carbondioxide, now contained within the liquid effluent stream 117, is reducedand removed from the liquid effluent stream 117 to regenerate thesolvent (e.g., a lean, low CO₂ containing solvent). The pressure of thestripper is sufficient to flow the CO₂ to the anode gas oxidizer (AGO).The regenerated solvent is discharged from an output end 116 of thestripper column 115, cooled, and fed to the absorber column 114. Thecarbon dioxide removed from the liquid effluent stream 117 in thestripper column 115 is output from another portion of the strippercolumn 115 (e.g., an end opposite to the end at which the output end 116is provided), as the CO₂-rich stream 118.

In some examples, the solvent used in the absorber column 114 may bemixtures of dimethyl ethers of polyethylene glycol. A physical solventis preferred to maximize the ease of solvent regeneration without theuse of additional heat.

In some examples, the solvent used in the absorber column 114 may be anamine solution. Some amine solutions, such as those developed by CarbonClean Solutions, use additives which have been optimized to reduce theamount of heat required for regenerating the amine while still absorbingthe CO₂ at low pressure. Absorption at low pressure is desirable tominimize the blower power required. Regeneration with less heat isdesired to minimize the amount of fuel (second portion 137) sent to theAGO, maximizing the amount of hydrogen recycled and/or exported.

A ChemCad heat and material balance was performed for the hightemperature molten carbonate fuel cell system 100 shown in FIG. 2. Asummary of the key streams from this balance are shown in FIG. 5. Theheat and material balance showed an increase in the electricalefficiency of the fuel-cell from 47% to 54% on an a lower heating value(LHV) basis. In this case, almost all of the fuel cell waste heat wasused for feed heating, amine regeneration and air pre-heating. Thisallows approximately 55% of the anode exhaust to be recycled. In thebalance shown, the amount of hydrogen recycled was chosen such that thatthe high temperature fuel cell system 100 could be balanced without anexternal source of heat. If an external heat source is available (suchas low pressure steam), more of the anode exhaust gas may be recycled,thereby further increasing the efficiency. Although not shown in thefigures, fuel such as natural gas, may be added to the anode exhaustgoing to the AGO (second portion 137). This allows more hydrogen to beexported, while maintaining the required heat input into the system forheat balance purposes, but may not provide a benefit when the anodeexhaust is recycled.

In addition, carbon dioxide capture from the system is very efficient,since a high purity carbon dioxide stream is generated from the strippercolumn 115. Although the carbon dioxide is at low pressure, the highpurity would allow liquefaction of the carbon dioxide at lower pressureand/or at a higher temperature with less refrigeration power required.The amount of carbon dioxide captured is limited for a molten carbonatefuel cell operation, since some carbon dioxide in the cathode exhaust isrequired for proper fuel cell operation.

In other implementations, the high temperature fuel cell system can beused for hydrogen export instead of recycling hydrogen to the fuel cell.Exporting the hydrogen may raise the overall efficiency of the hightemperature fuel cell system to around 60%. Referring to FIG. 3, in someexamples, the resultant gas (i.e., the gas composed of hydrogen and ahydrocarbon having carbon dioxide removed therefrom in the absorbercolumn 114 is output from the absorber column 114 in a first portion 122provided to the stream 124 and/or in a second portion 113 provided to apressure swing adsorption (PSA) system 120 configured to separate thecarbon dioxide and other impurities from the hydrogen. In operation, allof the resultant gas may be provided as the first portion 122, all ofthe resultant gas may be provided as the second portion 113, or part ofthe resultant gas may be provided as the first portion 122 with theremainder of the resultant gas provided as the second portion 113. Priorto entering the PSA system 120, the second portion 113 is pressurized ina compressor 119. The pressurized second portion 113 (i.e., a PSA feed)is input into the PSA system 120, in which carbon dioxide and otherimpurities, such as unconverted methane and CO, are removed from the gasstream, leaving pure hydrogen. The carbon dioxide, unconverted methane,and water are discharged from the PSA system 120 as a PSA output stream125 that combines with the first portion 122 to form the stream 124. Apure hydrogen product stream 123 is also output from the PSA system 120.PSA is based on a physical binding of gas molecules to adsorbentmaterial, with the binding force depending on the gas component, type ofadsorbent material, partial pressure of the gas component and operatingtemperature. Separation, and ultimately purification of the feed gas, isbased on differences in binding forces to the adsorbent material. Highlyvolatile components with low polarity, such as hydrogen, are weaklyadsorbed, while molecules such as CO₂, CO, hydrocarbons, N₂, and watervapor have strong binding affinity to the adsorbent. Consequently, theseimpurities are adsorbed from a H₂-containing stream, and high purityhydrogen is recovered. In one implementation, product stream 123comprises at least 95 mole % hydrogen. In another implementation,product stream 123 comprises at least 98 mole % hydrogen. In yet anotherimplementation, product stream 123 comprises at least 99 mole %hydrogen. In yet another implementation, product stream 123 comprises atleast 99.99 mole % hydrogen.

The product stream 123 may be used to produce higher hydrocarbons suchas methanol, other alcohols, or liquids from a Fischer-Tropsch (FT)reaction, especially if no shift reactor is used.

Providing the resultant gas from which carbon dioxide is removed in theabsorber column 114 to the PSA system 120 is a beneficial configurationfor hydrogen coproduction since a much smaller, more concentratedhydrogen stream is compressed for feeding to the PSA system 120. Thissmaller stream consumes much less compression power and increases thehydrogen recovery in the PSA system 120. In other words, theimplementation of FIG. 3 allows for hydrogen co-production withsubstantially lower parasitic power loads. This configuration reducescompression power by about 75% (for the same amount of hydrogen). Inaddition, the capital costs associated with the compressors and the PSAare also reduced, as the compressors and the PSA can be made smallerthan in a conventional system where the anode exhaust is sent to the PSAwithout CO₂ removal.

In the system of FIG. 1, FIG. 2, or FIG. 3, carbon dioxide from theCO₂-rich stream 118 may be captured. Thus, the systems of FIG. 1-3 maybe used to co-produce carbon dioxide, while the system of FIG. 3 may befurther used to co-produce high purity hydrogen.

The operation of an internally reforming molten carbonate fuel cell(MCFC) with carbon dioxide transfer is shown in FIG. 4. Hydrocarbon fuel(in this example, natural gas) and steam flow into an indirect internalreformer where it is partially reformed according to the followingequation:

CH₄+2H₂O→4H₂+CO₂  (1)

The partially reformed fuel then enters the anode of the fuel cell,where it is further reformed by a direct internal reforming catalyst(DIR catalyst) provided within the anode compartment and exposed to theelectrolyte of the fuel cell according to the equation:

H₂+CO₃ ²⁻→H₂O+CO₂+2e ⁻  (2)

Air and carbon dioxide recycled from the anode are supplied to thecathode. Thus, in Equation (3), the CO₃ ^(═) is produced by the cathodeaccording to the equation:

½O₂+CO₂+2e ⁻→CO₃ ²⁻  (3)

The electrons travel through an external circuit from the anode to thecathode, providing electrical power (DC power).

In systems including a MCFC, the anode exhaust has a very high carbondioxide content, since carbon dioxide from the cathode is transferred tothe anode during normal operation (see FIG. 4). This limits the amountof anode exhaust that can be recycled unless the carbon dioxide in theanode exhaust that is being recycled is removed, because the carbondioxide build-up in the anode feed reduces the voltage produced by thefuel cell to an unacceptable level with a relatively small amount ofrecycle. In systems including other high temperature fuel cells, such asa solid oxide fuel cell (SOFC), only oxygen is transferred from thecathode to the anode during normal operation. During normal operation ofhigh temperature fuel cells including MCFCs and SOFCs, the anode exhaustis at a low pressure, for example, a pressure of approximately 15 to 18psia, which makes it difficult to remove carbon dioxide from the anodeexhaust.

As utilized herein, the terms “approximately,” “about,” “substantially”,and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numerical ranges provided. Accordingly, these terms shouldbe interpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of the claims.

The terms “coupled,” “connected,” and the like as used herein mean thejoining of two members directly or indirectly to one another. Suchjoining may be stationary (e.g., permanent) or moveable (e.g., removableor releasable). Such joining may be achieved with the two members or thetwo members and any additional intermediate members being integrallyformed as a single unitary body with one another or with the two membersor the two members and any additional intermediate members beingattached to one another.

References herein to the positions of elements (e.g., “top,” “bottom,”“above,” “below,” etc.) are merely used to describe the orientation ofvarious elements in the Figures. It should be noted that the orientationof various elements may differ according to other exemplary embodiments,and that such variations are intended to be encompassed by the presentdisclosure.

It is important to note that the construction and arrangement of thevarious exemplary embodiments are illustrative only. Although only a fewembodiments have been described in detail in this disclosure, thoseskilled in the art who review this disclosure will readily appreciatethat many modifications are possible (e.g., variations in sizes,dimensions, structures, shapes and proportions of the various elements,values of parameters, mounting arrangements, use of materials, colors,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter described herein. Forexample, elements shown as integrally formed may be constructed ofmultiple parts or elements, the position of elements may be reversed orotherwise varied, and the nature or number of discrete elements orpositions may be altered or varied. The order or sequence of any processor method steps may be varied or re-sequenced according to alternativeembodiments. Other substitutions, modifications, changes and omissionsmay also be made in the design, operating conditions and arrangement ofthe various exemplary embodiments without departing from the scope ofthe present application. For example, the heat recovery heat exchangersmay be further optimized.

What is claimed:
 1. A fuel cell system for removing carbon dioxide fromanode exhaust gas, the fuel cell system comprising: a fuel cell havingan anode configured to output an anode exhaust gas comprising hydrogen,carbon monoxide, carbon dioxide, and water; a shift reactor configuredto receive a first portion of the anode exhaust gas and to perform awater-gas shift reaction to produce an output stream primarilycomprising hydrogen and carbon dioxide; an anode gas oxidizer; and anabsorption system configured to receive the output stream from the shiftreactor, the absorption system comprising: an absorber column configuredto absorb the carbon dioxide from the output stream in a solvent and tooutput a resultant gas comprising hydrogen and a hydrocarbon that is atleast partially recycled to the anode; and a stripper column configuredto regenerate the solvent and to output a carbon dioxide-rich stream,wherein: the anode gas oxidizer is configured to receive and oxidize ananode gas oxidizer input stream and at least a portion of the carbondioxide-rich stream; and the anode gas oxidizer input stream comprisesone of a second portion of the anode exhaust gas or a portion of theoutput stream from the shift reactor.
 2. The fuel cell system of claim1, wherein another portion of the carbon dioxide-rich stream output bythe stripper column is captured.
 3. The fuel cell system of claim 1,wherein the solvent comprises an amine solution.
 4. The fuel cell systemof claim 1, wherein the solvent comprises mixtures of dimethyl ethers ofpolyethylene glycol.
 5. The fuel cell system of claim 1, furthercomprising: a first heat exchanger located upstream of the shiftreactor, the first heat exchanger configured to cool the first portionof the anode exhaust gas; and a second heat exchanger located downstreamof the shift reactor, the second heat exchanger configured to cool theoutput stream.
 6. The fuel cell system of claim 5, further comprising: awater recovery system downstream of the second heat exchanger, the waterrecovery system configured to recover water from the cooled outputstream and to recycle the recovered water to the anode.
 7. The fuel cellsystem of claim 6, wherein the anode is configured to receive a fuel gascomprising the resultant gas from the absorber column, the recoveredwater from the water recovery system, and a hydrocarbon streamcomprising at least one of methane, natural gas, propane or otherhydrocarbon.
 8. The fuel cell system of claim 1, further comprising apressure swing adsorption system configured to receive at least aportion of the resultant gas from the absorber column and to separatehydrogen from the resultant gas.
 9. The fuel cell system of claim 8,wherein: the pressure swing adsorption system is configured to output afirst stream comprising the hydrogen and a second stream comprisingcarbon dioxide and the hydrocarbon; and the second stream is recycled tothe anode.
 10. The fuel cell system of claim 1, wherein the anode gasoxidizer is further configured to receive a pre-heated air stream. 11.The fuel cell system of claim 10, further comprising a cathodeconfigured to output a cathode gas, wherein: the cathode exhaust gas isconfigured to heat an air stream to produce the pre-heated air stream;and the cathode exhaust gas is configured to heat the stripper bottom toproduce a lean solvent.
 12. The fuel cell system of claim 1, furthercomprising a cathode configured to output a cathode gas, wherein thecathode exhaust gas is configured to heat fuel gas upstream of theanode.
 13. A method of removing carbon dioxide from fuel cell anodeexhaust gas, the method comprising: outputting anode exhaust gascomprising hydrogen, carbon monoxide, carbon dioxide, and water from theanode; receiving a first portion of the anode exhaust gas in a shiftreactor; receiving an anode gas oxidizer input stream in an anode gasoxidizer; performing a water-gas shift reaction in the shift reactor toproduce an output stream primarily comprising hydrogen and carbondioxide; receiving the output stream from the shift reactor in anabsorption system comprising an absorber column having a solvent thereinand a stripper column; absorbing the carbon dioxide from the outputstream in the solvent and outputting, from the absorber column, aresultant gas comprising hydrogen and a hydrocarbon that is at leastpartially recycled to the anode; regenerating the solvent andoutputting, from the stripper column, a carbon dioxide-rich stream; andoxidizing the anode gas oxidizer input stream and at least a portion ofthe carbon dioxide-rich stream to produce an oxidant gas, wherein theanode gas oxidizer input stream comprises one of a second portion of theanode exhaust gas or a portion of the output stream from the shiftreactor.
 14. The method of claim 13, further comprising capturinganother portion of the carbon dioxide-rich stream.
 15. The method ofclaim 13, wherein the solvent comprises an amine solution.
 16. Themethod of claim 13, wherein the solvent comprises mixtures of dimethylethers of polyethylene glycol.
 17. The method of claim 13, furthercomprising: cooling the first portion of the anode exhaust gas prior toentering the shift reactor; and cooling the output stream of the shiftreactor.
 18. The method of claim 17, further comprising: recoveringwater from the cooled output stream; and recycling the recovered waterto the anode.
 19. The method of claim 18, further comprising receiving afuel gas at the anode that comprises the resultant gas from the absorbercolumn, the recovered water from the water recovery system, and a gasstream comprising at least one of methane or natural gas.
 20. The methodof claim 19, further comprising heating the fuel gas, upstream of theanode, using cathode exhaust gas.
 21. The method of claim 13, furthercomprising: separating hydrogen from the resultant gas using a pressureswing adsorption system.
 22. The method of claim 21, further comprising:outputting, from the pressure swing adsorption system, a first streamcomprising hydrogen; outputting, from the pressure swing adsorptionsystem, a second stream comprising carbon dioxide and the hydrocarbon;and recycling the second stream to the anode.
 23. The method of claim13, further comprising: heating an air stream using cathode exhaust gasto form a pre-heated air stream; and providing the pre-heated air streamto the anode gas oxidizer.